Field of the Invention
The present invention relates to an optical data transmitting apparatus and a method for an optical data transmission system using optical fiber.
Recently, in optical fiber data transmission, intensity modulation direct detection systems have been widely used and wavelength-multiplexing technology has been employed for increased data transmission capacity. In this type of system, a combination of group velocity dispersion (GVD) and self phase modulation (SPM) occurring in an optical fiber causes waveform distortion, resulting in degraded data transmission quality.
GVD is a phenomenon in which propagation speed through optical fiber varies with wavelength. Modulated signals suffer from waveform distortion because of its wide signal spectrum. Recent technology provides a compensation element for waveform distortion due to GVD. Such a compensation element, however, is expensive. Thus, there have been proposals for modulated signals which are less affected by GVD. The narrower the signal spectrum, the less the signal is affected by GVD. Thus, improvement may be expected if the signal spectrum is narrowed. Optical duobinary modulation is one approach.
JP-A 8-139681, which is deemed to be the Japanese equivalent of U.S. Pat. No. 5,543,952 (Yonenaga et al.), and K. Fukuchi et al. OFC '97 Technical Digest, ThH3, 1997 teach optical data transmission utilizing such optical duobinary modulation. Duobinary signals fall within a category of partial response signals. Three-level duobinary signals are known, and have three levels of amplitude. According to the optical duobinary modulation, three levels are allocated to three states “+1”, “0 ”, “−1” using phase and amplitude of light. The state “0” indicates that the intensity level is zero. The state “+1” and “−1” indicate that the intensity level is one, but the phases are inverted from each other. At the receiver, this optical duobinary modulated signal is decoded into the original two-level digital signal by normal direct detection. During propagation through optical fiber, the signal band is left in a compressed state and waveform distortion due to GVD is low.
SPM is another cause of degradation of data transmission quality. SPM is a phenomenon in which phase modulation is applied to a signal as its light intensity varies with time. This problem is unavoidable if intensity modulation is employed. The above-mentioned optical duobinary signal is the same as the ordinary intensity modulated signal in that it uses light intensity. Thus, suppression of SPM cannot be expected.
SPM increases signal spectrum bandwidth although it does not alter the signal waveform, thereby limiting the density of wavelength multiplexing. SPM cooperates with GVD to induce a combined SPM+GVD effect.
As one approach to reduce SPM, polarization modulation has been proposed. In polarization modulation, light signals are allocated to different polarization states of light. As the overall intensity is unaltered, the effect of SPM is less, thus suppressing degradation of data transmission quality. JP-A 1-208920 (Fukaya) discloses an optical fiber data transmission employing such polarization modulation.
The SPM+GVD effect in question is a phenomenon in which during propagation through an optical fiber, SPM occurs concurrently with the occurrence of waveform distortion, and thus the dispersion compensation technique cannot restore the waveform. If SPM occurs concurrently with waveform distortion, the light signal is subjected to frequency chirping. The presence of GVD causes the chirping to induce further waveform distortion. This further waveform distortion induces new SPM. Thus, the waveform distortion is different from distortion in the absence of SPM, and this is the reason why the dispersion compensation technique is ineffective to restore the waveform distortion under the presence of SPM. The effect of GVD+SPM is thus considered to be the major obstacle to the extension of data transmission distances in optical fiber data transmission to any distance exceeding 1000 kilometers.
From preceding description, it is to be understood that the conventional duobinary signal is ineffective against SPM, and polarization modulation is ineffective to prevent GVD, although the former is effective against GVD and the latter is effective against SPM.